The discharge of granular and other dry materials from railroad hopper cars has long presented a variety of problems. Since the 1940's, pressure-type pneumatic conveyors have been developed in which a compressor (blower) is attached to a convey line which branches at the railcar. One branch of the convey line is connected to pressurize the railcar and a second branch is connected to a discharge outlet of the pressurized railcar to convey material away from the railcar. Government regulations in the United States limit the pressurization of railcars to less than 15 PSI, thus effectively limiting the rate of off-loading material from these railcars.
Pressurized discharge systems from the late 1940's until the mid 1980's typically used convey lines of 4" or 5" in diameter. While demand for increased off-loading rates was present, the 15 PSI railcar pressurization limits prevented any significant increases in line sizes or off-loading rates. Finally, in the late 1980's, some discharge and conveyor systems used large enough blowers to increase discharge line sizes to 8" diameter and transport convey line sizes to 12" diameter. A bypass and throttling valve was added to regulate the amount of pressurized air entering the railcar discharge convey line vs. the air in the transport convey line.
While these changes improved the off-loading rates, a number of problems remained. In order to pressurize the railcar to approximately 15 PSI and, at the same time, to provide the volume of air needed to efficiently move the product through an 8" discharge convey line and thence through a considerable length of 12" transport convey line, large blowers were needed which required high horsepower motors, which are very expensive to purchase and to operate. In addition to the normal requirements of railcar pressurization and product conveyance, prior art blowers needed to be sized to compensate for the inefficiency and the relatively large pressure drop across the bypass throttling valve where flow of the transport convey air flow is bottlenecked. For example, typical discharge and transport convey line pressures are 6 PSI as compared to the 14+ PSI needed to pressurize the railcar. Some single blower systems have used motors of 500 horsepower, for example. Motors of this size do not typically run on ordinary voltages, e.g., a 500 horsepower motor is typically designed to utilize 6600-volt power, which is not readily available in a normal plant. Finally, it is very difficult to balance the volume of air entering the railcar vs. the volume of air in the discharge convey line as well as the transport convey line. With too much pressure drop between the railcar and the discharge convey line, material is moved too fast through the discharge line. This increases the wear on system components, and, if the product being conveyed is degradable, too much speed will adversely affect the product. By contrast, with too little pressure drop, material movement is too slow, allowing partial or total blockage of the discharge chutes and discharge and transport convey lines to result. In addition to the bypass throttling valve, other expensive and relatively complicated pressure regulators were required between the blower and the railcar as well as between the blower and the discharge convey line to regulate air volume and pressure. Unless unloading personnel are well trained and vigilant, proper air flow regulation via these regulators is often intermittent at best.
Accordingly, a need exists for an improved pressurized railcar dry material discharge and conveyor system and method. Such an improved system and method should preferably achieve discharge rates comparable to known systems equipped with throttling valves but with smaller compressors driven by lower horsepower motors. Finally, the improved system and method should preferably eliminate the bypass throttling valve with its inherent losses and balancing problems. Air flow regulation should be simplified as well.